3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3GPP to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
The most common form of UMTS makes use of Wideband Code Division Multiple Access (WCDMA), which is an air interface standard that is a compulsory feature of any wireless device of the UTRAN. High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), together referred to as High Speed Packet Access (HSPA), are mobile communication protocols that were developed to cope with higher data rates than original WCDMA protocols were capable of.
FIG. 1a illustrates a radio access network with an RBS 101 that serves a UE 103 located within the RBS's geographical area of service, called a cell 105. In UMTS, a Radio Network Controller (RNC) 106 controls the RBS 101 and other neighboring RBSs, and is, among other things, in charge of management of radio resources in cells for which the RNC is responsible. The RNC is in turn also connected to the core network (not illustrated). FIG. 1b illustrates a radio access network in an LTE system. An eNodeB (eNB) 101a serves a UE 103 located within the RBS's geographical area of service or the cell 105a. The eNodeB 101a is directly connected to the core network (not illustrated). The eNodeB 101a is also connected via an X2 interface to a neighboring eNodeB 101b serving another cell 105b. 
During the last few years, cellular operators have started to offer mobile broadband based on WCDMA/HSPA. Further, fuelled by new wireless devices designed for data applications, the end user performance requirements are steadily increasing. The large uptake of mobile broadband has resulted in that traffic volumes that need to be handled by the HSPA networks have grown significantly. Therefore, techniques that allow cellular operators to manage their spectrum resources more efficiently are of large importance.
Some techniques which make it possible to improve the downlink performance are 4-branch Multiple-Input-Multiple-Output (MIMO), multi-flow communication, and multi-carrier deployment. Since improvements in spectral efficiency per link are approaching theoretical limits, the next generation technology is about improving the spectral efficiency per unit area. In other words, the additional features for HSDPA need to provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks. Currently 3GPP has been working on this aspect of using heterogeneous networks.
A homogeneous network is a network of radio network nodes, such as RBSs, NodeB, Remote Radio Heads (RRH), and Remote Radio Units (RRU), in a planned layout and a collection of user terminals. In the homogeneous network all radio network nodes have similar transmit power levels, antenna patterns, and receiver noise floors, as well as similar backhaul connectivity to the data network. In other words they are all belonging to a same base station power class. For example, all of them are either high power nodes (HPN) or low power nodes (LPN). An example of a HPN is a wide area RBS serving a macro cell. An example of a LPN is a local area RBS serving a pico cell. In other words a homogeneous network is a single tier system. Moreover, all RBSs offer unrestricted access to user terminals in the network, and serve roughly the same number of user terminals. Current wireless systems such as WCDMA, HSPA, and LTE fall under this category.
In heterogeneous networks several LPNs 202 such as micro, pico, femto, or relay base stations are deployed in addition to the planned or regular placement of HPNs 201 such as wide area RBSs serving macro cells 203, as shown in FIG. 2a. Therefore a heterogeneous network is at least a 2-tier system. Note that the power transmitted by these micro, pico, femto, or relay base stations is relatively small compared to that of macro base stations. A LPN may transmit at a power which can be up to 2 W, as compared to that of 40 W for macro base stations. The LPNs are often deployed to eliminate coverage holes in the homogeneous network. Hence they improve the capacity in hot-spots. Due to their lower transmit power and smaller physical size, LPNs can offer flexible site acquisitions.
The LPN cells in a cluster of heterogeneous nodes of a heterogeneous network may have different cell identifiers from that of HPN cells which means that they are viewed as different cells. Alternatively, they can have same cell identifiers as that of HPN cells. Such cells are sometimes referred to as soft, shared, or combined cells, or cluster with common cell identifiers.
FIG. 2b shows the heterogeneous network where LPNs 202 and HPNs 201 create separate cells 204 and 203 respectively, i.e. with different cell identifiers illustrated by the dotted cell 206 and 205 overlaid the illustration of the actual cell coverage 204 and 203. Simulations show that using LPNs in a macro cell offers load balancing, hence enabling large gains in system throughout as well as cell edge user throughput. One disadvantage with each cell creating a different cell is that a UE needs to do soft handover when moving from an LPN cell to a HPN cell or to another LPN cell. Hence higher layer signaling is needed to perform the handover.
FIG. 2c illustrates a heterogeneous network where LPN cells are part of the HPN cells, i.e. share cell identifier illustrated by the dotted cell 205 overlaid the illustration of the actual macro cell coverage 207. This set-up avoids frequent soft handovers and hence higher layer signaling. In the deployment of FIG. 2c, all the nodes are coupled to a central node, which in this case is a HPN 201. In a typical deployment scenario the LPNs are connected to a central controller via a high speed data link. The central controller in the combined cell takes responsibility for collecting operational statistics information of network environment measurements. The decision of what nodes that should transmit to a specific UE is made by the central controller, possibly based on information provided by the UE. The cooperation among various nodes is instructed by the central controller and implemented in a centralized way. The central controller is one of the network nodes, e.g. the HPN.
Even though large gains in terms of average sector throughput are achieved with the introduction of LPNs, the interference structure becomes more complex in heterogeneous networks. For example when a UE is served by an LPN, individual UE link throughput is impacted due to the interference caused by the HPN. FIG. 3a shows the link performance when a UE which is scheduled by an LPN experiences a strong interference from a HPN such as a macro RBS which is serving another UE. The interference due to other nodes than the interfering HPN is modeled as white noise. The diagram of FIG. 3a illustrates the performance degradation that occurs when the UE is in the vicinity of a strongly interfering HPN or macro node. In the diagram, the values for link throughput is plotted for different interference situations, given by an loc value that determines how strong the interfering signal from the macro node is compared to the signal strength from the serving cell. loc=0 dB means that the interfering signal is equal to the serving cell signal, and loc=20 dB means that the interfering signal is 20 dB stronger than the serving cell signal. The performance loss is in the range of 100% at high geometries, i.e. for the highest value of loc.
Overview of Network Assisted Interference Cancellation (NAIC)
Range expansion is a technique in heterogeneous networks where user terminals are offloaded to LPNs even though they experience better downlink reception from the HPN or the macro RBS, thereby achieving load balancing gains. However, the performance of user terminals which are connected to LPNs is then impacted due to strong interference from the HPN. The HPN is in this case referred to as the aggressor node. It can be seen that significant performance gains can be achieved if the UE knows about signal format information of the interfering signals and thus can cancel the interference. This method is referred to as NAIC. FIG. 3b shows the link performance in terms of throughput when the network signals scheduling information for an interfering transmission from the aggressor HPN in accordance with a NAIC procedure. In the simulation behind the measurement results depicted in the diagram of FIG. 3b, the interfering signal is re-constructed at the UE receiver and the interference is then removed prior to detecting the serving cell signal. The macro node interference is in this example scenario 20 dB stronger than the LPN desired signal, i.e. loc=20 dB.
FIG. 3b shows a simulated link performance when the network signals the scheduling information of the interferer. In the simulation, the interference signal was re-constructed at the UE receiver and the interference was removed prior to detecting the serving cell signal. It can be seen from FIG. 3b that significant performance gains can be achieved if the UE knows the signal format information about the interfering signals.
For HSDPA, a transport layer channel, High-Speed Downlink Shared Channel (HS-DSCH), is implemented by three physical layer channels: High Speed-Shared Control Channel (HS-SCCH), Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH), and High Speed-Physical Downlink Shared Channel (HS-PDSCH). The HS-SCCH informs the UE that data will be sent on the HS-DSCH, 2 slots ahead. The HS-DPCCH carries acknowledgment information and current channel quality indicator (CQI). This is then used by the base station to calculate how much data to send to the UE on the next transmission. The HS-PDSCH is the channel to which the above HS-DSCH transport channel is mapped that carries actual user data. The Common Pilot Channel CPICH carries the broadcasted pilot signal identifying the NodeB cell. FIG. 4 is a signaling diagram illustrating the message sequence used for conveying the scheduling information or the network assistance information of the aggressor NodeB 402 to the wireless device 403 served by NodeB 401 in a HSPA network. The scheduling information may be conveyed by a common HS-SCCH order from the aggressor NodeB 402. Alternatively, the network assistance information may be conveyed through a broadcast channel. The channels conveying network assistance information such as the HS-SCCH may be referred to as network assisted control channels.
The network assisted control channel may contain either of the following network assistance information:                1. Scheduling information for the interfering downlink transmission by the aggressor node. In one solution, the HS-SCCH order consists of bits indicating that it is an order for informing about the scheduling information from the aggressor node. The scheduling information comprises modulation, transport block size information, and spreading codes, i.e. orthogonal variable spreading factor (OVSF) codes used at the scheduling of the interfering transmission. The scheduling information may additionally comprise pre-coding and rank information when the aggressor node applies MIMO transmissions.        2. An identifier of the UE to which the aggressor node has scheduled a transmission that interferes with the LPN nodes transmission to the victim UE. In this case, the aggressor node conveys the identifier of a scheduled UE such that the victim UE can decode the HS-SCCH of the aggressor node directed to this UE and thereby retrieve the corresponding scheduling information of the interfering transmission.        
Alternatively, the network assistance information may be conveyed through a broadcast channel. The channels conveying network assistance information such as the HS-SCCH may be referred to as network assisted control channels.
Instead of sending the broadcast control channel, the RNC can pre-configure the NodeB and the UE with restricted resources. For example, it can restrict the NodeB to schedule only a subset of channelization codes, a subset of modulation codes and/or a subset of Transport Block (TB) sizes in specific Transmission Time Intervals (TTIs). This way, the UE knows the modulation and channelization code set and/or TB size of the interferer, and it can reconstruct the interference signal and subtract it from the received signal, thereby at least partially removing the interference. With this NAIC using restricted resources, the cancellation may be performed more robustly.
FIG. 5a is a signaling diagram illustrating the message sequence during typical data call set up between NodeB and the UE. From the Common Pilot Channel (P-CPICH), the UE estimates the channel and computes the CQI. This information along with Hybrid Automatic Repeat reQuest (HARQ) ACKnowledgement/Negative ACKnowledgement (ACK/NACK) information is reported to NodeB using High Speed-Dedicated Physical Control Channel (HS-DPCCH). The minimum periodicity of HS-DPCCH is one subframe (2 ms) and the actual value of this period is configured by the RNC through Radio Resource Control (RRC) signaling.
Once the Node-B receives this information, it allocates the required channelization codes, modulation and coding to the UE after scheduling. This information is conveyed to UE by High Speed-Shared Control Channel (HS-SCCH). Once the UE detects the HS-SCCH, downlink transmission starts through data traffic channel using High Speed-Physical Downlink Shared Channel (HS-PDSCH).
The conventional structure of HS-DPCCH for a single carrier is illustrated in FIG. 5b, showing how the HARQ ACK/NACK information and the CQI are alternatingly signaled. It should be noted that CQI reporting is conveyed using 5 bits representing a specific modulation, a specific transport block size (code rate) and a number of channelization codes. These 5 bits are coded using Reed Muller code and produce a 20-bit code word, i.e. the 5 bits are mapped to (20, 5) code block. For single carrier, the 20 encoded bits are spread using SF-256 and transmitted to the NodeB. For multi carrier, SF-128 is used in some cases.
In view of the above, a scheduling is desirable that efficiently accounts for the interference cancellation (IC) at the US served by an LPN.